Regular and Modulated Tantalum Seleno-Chlorides: Synthesis and Structure of Chain Compounds Ta4Se16(TaCl6)2 and...

The first tantalum seleno-chlorides were synthesized in two modifications: Ta4Se16(TaCl6)2 (1) and an incommensurately modulated (TaSe4)18(TaCl6)10 (2). Both phases contain infinite cationic chains (TaSe4)x+. 1 was obtained by heating a mixture of TaSe3, Se, and Ta2Cl10 in a two-zone ampule with a 350/340 °C gradient. 2 was obtained in a similar way using TaSe2 instead of TaSe3. The crystal structure of compound 1 (space group P-1, unit cell parameters a = 12.2375(3) Å, b = 12.7820(4) Å, c = 13.6349(4) Å, α = 66.633(1)°, β = 66.310(1)°, γ = 78.725(1)°, Z = 4, V = 1791.20(9) Å3) includes positively charged chains {TaSe4}0.5+ and molecular anionic complexes [TaCl6]-. In the chains {TaSe4}, the Ta···Ta distances vary in the sequence: ···3.184; 3.112; 3.152; 3.335 Å···. The crystal structure of compound 2 (orthorhombic cell, a = 3.1555(3) Å, b = 16.077(2) Å, c = 19.0654(8) Å, V = 967 Å3, modulation vector q = 0.278·a*) is incommensurately modulated. We established the X-centering of the orthorhombic cell, evident from the extinction pattern, and the helical pattern of {TaSe4} chains, confirmed by the reported structure solution in the 18×a supercell. The structure of 2 is characterized by an average Ta···Ta distance of 3.16 Å in the infinite chains {TaSe4}. In this work, we discuss the synthesis and characterization of compounds 1 and 2, their crystal structures, and the magnetic and resistivity measurements. The discussion of experimental data is supported by DFT estimation of enthalpies (T = 0).